Questions and answers: design of surface excavations 32 1instability mechanisms. These are then tabulated, as shown below.Slope dip direction Maximum slope angle Critical angle Critical mode
plane wedge(^000 90 55 55) wedge
015 90 57 57 wedge
(^030 90 63 63) wedge
045 75 70 70 wedge
060 75 65 65 wedge
(^075 90 52 52) wedge
090 90 45 45 wedge
105 90 38 38 wedge
(^120 90 35 35) wedge
135 35 35 35 plane
150 35 38 35 plane
165 35 40 35 plane
(^180 90 47 47) wedge
195 90 57 57 wedge
210 90 75 75 wedge
225 90 90 90 plane
240 90 90 90 plane
(^255 90 87 87) wedge
270 90 75 75 wedge
285 65 68 65 plane
(^300 65 60 60) wedge
315 90 57 57 wedge
330 90 53 53 wedge
345 90 52 52 wedge
All angles in degrees.
Toppling instability has not been included in this table because there
is little, if any, incidence of it in this rock mass. This may be readily
seen, because the two principal requirements for direct toppling - i.e.
two fracture sets which possess a near-vertical line of intersection, and a
horizontal fracture set - are not present. The only likelihood of toppling
instability is with those blocks formed by sets 1,2 and 3. For a friction
angle of 30" these blocks should suffer sliding instability on set 2 but, if
they are prevented from doing so by some form of 'keying', and at the
same time are sufficiently high in the direction of the intersection of sets
1 and 3 to form unstable blocks, then toppling may occur.
418.4 Prepare a similar table as in Q18.3 but for the case when the
friction angle is not known.
A18.4 In the case when the friction angle is unknown, we must select
a slope angle such that the kinematic feasibility of all instability modes
is prevented. This is the same as determining the maximum permissible
slope angle to prevent instability when the friction angle is zero. Again,
three overlays are required: one for each of the instability modes of plane